So far, as an airflow sensor placed in an intake air passage of an internal combustion engine or the like for measurement of air quantity, the principal tendency has been toward a thermal type for direct detection of mass air quantity. Recently, particular attention has been focused on a small-sized airflow sensor produced through the use of the semiconductor micromachining technique, for its high response and for its counterflow detection ability using the high response. As heater materials for the conventional small-sized thermal type airflow sensor, as exemplified in JP-A-8-54269 or 11-233303, there has been used a semiconductor material such as polysilicon or a metallic material such as platinum, gold, copper, aluminum, chromium, nickel, tungsten, permalloy (FeNi) or titanium. In addition, as stated in JP-A-11-233303, such a small-sized sensor is applicable not only to the flow rate sensor but also to a relative-humidity detecting sensor or a gas detecting sensor.
There is a problem which arises with the conventional small-sized sensor, however, in that the resistance of the heater resistor itself varies due to the heating of the heater resistor formed on a thin-wall portion or the thermal influence from the surroundings. For this reason, as exemplified in JP-A-11-233303, there has been known a technique in which a conductive film is form independently of the heater resistor to prevent the temperature distribution to local heating, thus suppressing the variations with the passage of time.